The invention relates to the field of friction and braking materials. More particularly, the invention relates to the field of friction materials comprising a fibrous structure permeated with a binding matrix.
Needling processes for forming fibrous preform structures for use in composite structures have been known for many years. U.S. Pat. No. 3,772,115 to Carlson et al. describes a process whereby several fibrous layers may be needled together simultaneously or in a series of needling steps. The Carlson et al. needling process involves repeatedly driving a multitude of barbed needles into the fibrous layers. The barbed needles displace fiber within the layers which causes the layers to adhere into a coherent structure. The structure may be incrementally formed by adding layers in a series of needling steps if the final structure is too thick to allow the needles to pass all the way through. The fibrous layers comprise carbon or graphite fabric, or precursors thereof. A fibrous preform structure formed according to the process may be further processed into a carbon/carbon composite structure by deposition of a carbon matrix within the fibrous preform structure that binds the fibers together. The Carlson et al. process may be used to form various composite structures, including carbon/carbon brake disks.
A similar process is disclosed in Great Britain Patent Specification 1,549,687, published Aug. 1, 1979. This also discloses a process for forming a carbon/carbon composite material. The fibrous layers may be comprised of oxidized polyacrylonitrile cloth which are needled together in a series of needling steps. In one example, the process was used to form a carbon brake disk.
A more recent process is disclosed by U.S. Pat. No. 4,790,052 to Pierre Olry. The goal is to produce a fibrous preform structure having a high degree of uniformity. This purportedly is accomplished by needling superposed layers together with a "uniform density" of needling throughout the thickness of the article. The initial depth of penetration is determined as a function of the number of layers to be traversed by the needles, for example about twenty layers. The Olry et al. process attempts to keep this depth constant throughout formation of the fibrous preform structure by lowering the fibrous structure away from the needles a distance equal to the thickness of a needled layer each time a new layer is added.
U.S. Pat. No. 4,955,123, issued Sep. 11, 1990 and PCT Publication WO 92/04492, published Mar. 19, 1992, both to Lawton et al., describe a process whereby a brake disk is formed by needling together sectors of an annulus. The fibrous structure is lowered the thickness of a needled layer each time a new layer is added. U.S. Pat. No. 5,338,320, issued as a continuation in part from the '123 patent on Feb. 14, 1995, to Smith et al. describes a process wherein outer layers of a preform are "enriched" with staple fiber in a needling process. Enriching the outer layers in the Smith et al. process apparently increases mechanical strength in the outer layers and improves wear characteristics of a resulting disk brake.
In the fibrous preform art, the displaced fibers generated by the needling process are referred to as "Z-fibers" since they are generally perpendicular to the layers comprising a fibrous preform structure. The Z-fiber distribution throughout a brake disk can have a profound effect on disk wear life and on performance of the brake disk in slowing or stopping an aircraft. A brake disk having improved resistance to wear while maintaining strength and rejected takeoff performance is generally desired.